Creating depth of field from Beauty+Depth passes (rendered in another software)
The Defocus node works in conjunction with the settings for the active camera.

You can use this node setup:



You have to check "Use Z-Buffer" so your depth image is used. If you omit the Add node all parts of the image where the depth image is black are sharp independent of your focal settings. I don't know why, maybe that's a bug.

You also have to set the "Distance" to 1.0 with your active camera selected under Properties > Camera Data > Depth of Field.

This is the result: